In one example of known liquid droplet discharging apparatuses for making the liquid material land on a work after the liquid material has separated from a nozzle, a plunger rod is disposed in a flow passage having a valve seat near an outlet communicating with the nozzle such that a lateral surface of the plunger rod is not in contact with the flow passage, and the liquid material is discharged from the nozzle by moving a fore end of the plunger rod to the valve seat for striking against the valve seat (Patent Document 1).
Also, as a technique of causing the liquid material to fly out in droplet form by quickly advancing a plunger and abruptly stopping the plunger without striking it against a valve seat, the applicant has previously proposed a liquid material discharging method and apparatus in which a liquid material discharging plunger having a fore end surface held in close contact with the liquid material is advanced at a high speed, and a plunger driving means is then abruptly stopped, whereupon the liquid material is discharged by application of inertial force to the liquid material (Patent Documents 2 and 3).
An ink jet printer is a liquid material discharging apparatus that discharges ink in the sate of a liquid droplet. The discharge amount of an ink droplet has been reduced year by year. Recently, an ink jet printer having the discharge amount of 30 pico-liter or less has also been provided. In such an ink jet printer discharging the ink droplet in a very small amount, the distance from a nozzle to a sheet of paper, called a nozzle-to-sheet distance, is as short as 1.0 mm to 1.5 mm. Further, an ink jet head is moved at a high speed of 500 mm to 2000 mm/sec (Patent Document 4).                Patent Document 1: PCT Japanese Translation Patent Publication No. 2001-500962        Patent Document 2: Japanese Patent Laid-Open Publication No. 2003-190871        Patent Document 3: Japanese Patent Laid-Open Publication No. 2005-296700        Patent Document 4: Japanese Patent Laid-Open Publication No. 2006-192590        